The present invention relates to improvements in apparatus for conveying foamed compositions, for example, to improvements in apparatus for conveying foam from one or more foam generators to a station or unit where the foam (or certain ingredients of the foam) must be applied to a stationary or running substrate.
It is already known to contact a running or stationary substrate with a continuous or discontinuous stream of foam which is produced in one or more foam generators and is conveyed to the path for the substrate by one or more conduits. Reference may be had, for example, to German Offenlegungsschrift No. 25 23 062. The application of foam to substrates is one of numerous attempts to apply minute quantities of liquids with a high degree of accuracy. This is desirable, for example, in connection with the application of chemicals (such as pigments, liquors or the like) to textile materials, webs of paper, cords, strands and like commodities. A typical example of a commodity which is to be provided with accurately controlled quantities of liquor in one or more colors is a carpet whose nap is to be dyed or patterned but whose base layer should remain free of contact with the dye or dyes. The chemicals are dissolved in a liquid carrier (such as water), and the liquid carrier is admitted into a foam generator to increase its volume as a result of admission of bubbles of air or another gas. In many instances, the foaming of liquids takes place in the presence of tensides. The aforementioned German publication discloses an apparatus which includes means for generating foam by resorting to a structure which is capable of varying the ratio of gaseous fluid and the ratio of liquid fraction in the foam. This enables the apparatus of the German publication to produce foams of desired consistency. Other attempts to produce foams include resort to dispersion turbines whose operation is analogous to that of the foam generator means in the Offenlegungsschrift. The latter further discloses a foam applicator which can be used to bring the foam in contact with a running substrate. Other prior publications which disclose means for conveying foam toward and for applying foam to a substrate include U.S. Pat. Nos. 3,969,780, 4,193,762 and 4,208,173. The U.S. Pat. No. 3,969,780 discloses two foam generators which are mounted in parallel and serve to apply foam directly to a running web or the like.
All of the heretofore known apparatus exhibit certain serious drawbacks which affect the quality of the dyed or otherwise treated substrates or other types of workpieces. The primary reason for the failure of conventional apparatus under circumstances when the treating composition or compositions must be applied with a very high degree of accuracy and reproducibility is that the rate of consumption of foam at a station where the foam is applied to a substrate or the like often fluctuates within a wide range. The reasons for such fluctuations are numerous and include, among others, the extent to which the substrate can absorb foams. Such ability, in turn, depends on a variety of parameters which cannot be considered in heretofore known apparatus wherein the foam generator simply turns out foam of selected consistency and discharges it for transport to the applicator. The aforementioned parameters include, among others, the extent to which the liquid fraction is foamed, the rate of disintegration or decay of foam, the composition of tenside or tensides (if any) which are present in the foam, the consistency of the substrate, and the nature of preliminary or preceding treatment of the substrate. The capacity of the substrate to absorb liquids can vary within a wide range. Heretofore known attempts to account for such characteristics of the substrate include the provision of a bypass on the way from the foam generator to the consumer. To this end (and assuming that the consumer includes a squeegee which is to distribute foam over the surface of the substrate and draws foam from a chamber which is adjacent thereto, e.g., in the interior of a rotary cylindrical screen), a first valve is installed ahead of the chamber in the screen and such valve closes to prevent further admission of foam into the chamber when the latter is filled to a predetermined extent. The bypass contains a second valve which opens when the first valve closes and vice versa. Thus, when the momentary requirements of the consumer are satisfied, the second valve opens and permits the oncoming foam to enter a conduit leading to the drain. This can entail enormous losses in chemicals including highly expensive chemicals which are often employed in dyeing and similar plants.